1. Field of the Invention
The present invention relates to a network switch.
2. Description of the Related Art
A chassis type switch and a box type switch are known as examples of a network switch. The chassis type switch includes a plurality of line cards within a chassis (casing).
There is a network switch of the type including an FDB (Forwarding Database) and executing frame transfer in accordance with the FDB. The term “FDB” implies a database for defining correspondence between a destination MAC (Media Access Control) address and an output port (i.e., an identifier of a line card and an identifier of the output port at a transfer destination in the case of the chassis type switch, or only an identifier of the output port in the case of the box type switch, or an identifier of a Link Aggregation Group (hereinafter abbreviated to an “LAG”)). The term “LAG” implies a technique of bundling a plurality of transmission lines together and virtually handling the bundled transmission lines as one transmission line. Plural ports set in the LAG are collectively handled as one port. In the chassis type switch, the FDB is installed in each line card.
The operation of frame transfer with the chassis type switch is described below as one example. When a unicast frame is received by the chassis type switch, the line card having received the unicast frame refers to the FDB, which is installed therein, and extracts the identifier of the line card and the identifier of the output port (or the identifier of the LAG) at the transfer destination corresponding to the destination MAC address of the received frame. The relevant line card then transfers the frame to the extracted transfer destination.
When the transfer destination (output port) extracted by referring to the FDB is the LAG, the identifier of the line card and the identifier of the output port at the transfer destination corresponding to a distributed ID, which is set in a predetermined manner, are extracted by referring to an LAG transfer destination table (i.e., an LAG-destined unicast transfer destination decision table) set for the relevant LAG. The LAG transfer destination table is a database for defining correspondence between the distributed ID and the respective identifiers of the line card and the output port at the transfer destination, and it is set for each LAG.
When the destination MAC address of the received frame is not registered in the FDB, a process of setting the relevant frame to be a destination unknown frame (“unknown”) and transferring the destination unknown frame to all ports of the relevant line card other than a reception port thereof and to all other line cards is executed (such a process is called “flooding”).
If a failure occurs in any port of the network switch, the unicast frame to be transferred to the port having caused the failure is not normally relayed and communication is interrupted. It is therefore desired that, if a failure has occurred in any port, switching-over to a backup route is performed in a time as short as possible.
In the related-art chassis type switch, if the occurrence of a failure in the line card within the device is detected, each FDB entry corresponding to the line card in which the failure has been detected is usually erased with software (such an erasing process is called “FDB flush”). When the FDB flush is executed, the unicast frame is subjected to the flooding as the destination unknown frame, and switching-over to the backup route is performed.
On the other hand, if a failure occurs in a port for which the LAG is set, the so-called degeneration of LAG is executed instead of the above-described FDB flush. More specifically, the LAG transfer destination table is rewritten with software such that any frame is not transferred to the port for which the failure has been detected (i.e., such that the output destination is switched over from the linked-down port to another linked-up port).
There are following patent documents as related-art information with regard to the invention of this application.
Japanese Unexamined Patent Application Publication No. 2008-136013
Japanese Unexamined Patent Application Publication No. 2010-263395
Japanese Unexamined Patent Application Publication No. 2009-027758
In the related-art network switch, however, if a failure occurs in a port for which the LAG is set, a problem arises in that about several hundreds milliseconds (ms) are taken from the detection of the port failure to change of the output destination (i.e., until rewrite of the LAG transfer destination table with software is completed), and that the communication is interrupted for a very long time. The reason will be described in detail below.
Let here consider the case where, in a chassis type switch 41 including three line cards 42a, 42b and 42c (LC1, LC2, and LC3), an LAG10 is set over two line cards 42b and 42c as illustrated in FIG. 5A.
When a unicast frame is input to the line card 42a (LC1), an FDB 43 installed in the line card 42a is first referred to for extraction of a transfer destination corresponding to a destination MAC address of the received unicast frame. Here, the transfer destination is assumed to be the LAG10.
Because the transfer destination is the LAG (i.e., the LAG10), an identifier of a line card and an identifier of an output port at the transfer destination corresponding to a distributed ID, which is set in a predetermined manner, are then extracted by referring to an LAG transfer destination table 44. Here, the transfer destination is assumed to be a port 1 of the line card 42b (LC2). The input unicast frame is transferred to the line card 42b, i.e., the transfer destination, and is output from the port 1 of the line card 42b. 
Here, it is assumed that, as illustrated in FIG. 5B, a failure (“Link Down”) has occurred in the line card 42b (LC2) of the chassis type switch 41. In such an event, all ports of the line card 42b (LC2) are regarded as causing failures.
In the related-art chassis type switch 41, the failure of the line card or the port is detected by a management card (not illustrated) within the chassis type switch 41. In the case assumed here, because the LAG (LAG10) is set for the line cards 42b and 42c (LC2 and LC3), the management card executes the degeneration of LAG, for example, by sending a control frame to all the line cards and by rewriting the LAG transfer destination table 44 in each line card. Communication of unicast frames destined for the line card 42b (LC2) is interrupted until the rewrite of all LAG transfer destination tables 44 is completed.
When the rewrite of the LAG transfer destination tables 44 is completed, the unicast frame is transferred to the line card 42c (LC3), whereby the switching-over of the route is performed.
Thus, in the chassis type switch 41, after detecting the failure of the port (or the line card) by the management card, the corresponding LAG transfer destination tables 44 in all the line cards have to be rewritten with software, and the operation of rewriting all the LAG transfer destination tables 44 take a time. Consequently, a time of about several hundred milliseconds is taken from the detection of the port failure to the switching-over of the route.
While the above description is made in connection with the chassis type switch 41 as one example, it is also required to rewrite an LAG transfer destination table with software in the box type switch if a failure has occurred in a port for which an LAG is set. As in the chassis type switch 41, therefore, a time of about several hundred milliseconds is taken from the detection of the port failure to the switching-over of the route. Because communication is interrupted for a time from the detection of the failure to the switching-over of the route, the switching-over time is desirably as short as possible.